typing fixes

[jorenham]

• replaced NoReturn with None, as NoReturn is the "bottom type" (i.e. the subtype of all types, the opposite of object, equivalent to typing.Never), and used to annotate functions that never return (e.g. because of a sys.exit or infinite loop).
• use MutableMapping instead of Mapping, which is closer to the actual dict it annotates, and so it can be used for mdit.options["mdformat"], as well.
• pass the missing generic type arguments to (Mutable)Mapping for the (_incorrectly contravariant) key and (covariant) value types. I assumed that the config values can only assume int (implicitly including its subtype bool), str, or Sequence[str]
• introduced a type-alias (without TypeAlias, which requires python<3.10) for the config options for the sake of DRY
• combined the {**_, **_} and the dict.update in update_mdit (kinda off-topic, but I couldn't help myself)

[jorenham]

I believe that CI failure is the same codecov issue that we ignored in #5

[csala]

@jorenham This has conflicts with the previous PR. Would you mind checking them?

[jorenham]

@jorenham This has conflicts with the previous PR. Would you mind checking them?

✔️

[csala]

Thanks @jorenham !